1. Field of the Invention
The present invention relates to a decoding technology, and more specifically, to a technology which decodes a signal coded by using a convolution code.
2. Description of the Related Art
A convolution code is frequently used as an error correction code in a field of the digital communication. FIG. 15 illustrates an example of a convolution encoder 300 which is usually used. The convolution encoder 300 obtains the convolution code of constraint length 7 and encoding rate 1/3, and is configured with plural adders 310 and plural delay elements 320 (D flip-flop illustrated as D in FIG. 15). Data line to be transmitted Input D is encoded by the convolution encoder 300 to three series of output signals Output A, Output B, and Output C. Meanwhile, while the convolution encoder 300 encodes Input D to three series of output signals, another configuration of convolution encoder may have other number than three as the number of series of output signals.
FIG. 16 illustrates aspects of the data line to be transmitted Input D and the output signals Output A, Output B, and Output C. The convolution encoder 300 outputs output of three bits (e.g. a0, b0, and c0) for input of one bit (e.g. d0).
In a communication system using the convolution code, a transmission side converts data line to be transmitted to the convolution code with an encoder, and modulates the code sequence obtained by the conversion to transmit it as modulated signal to a transmission line. A receiving side demodulates the modulated signal received from the transmission line to return it to the code sequence such as Output A, Output B, and Output C illustrated in FIG. 16, then further executes decoding processing. The Viterbi algorithm is well known as one of such decoding processing algorithms (non-Patent Document 1 (“The Viterbi Algorithm”, Forney, G. D, Jr. proceedings of IEEE, vol. 61, issue 3, pp 268-278) and non-Patent Document 2 (“The convolution code and the Viterbi decoding”, Hiroyuki Yashima, Torikeppusu)). The Viterbi algorithm compares the received code sequence with all the code sequence which may have been generated by the encoder of the transmission side (hereinafter, expected code sequence), selects the expected code sequence which is most close to the received code sequence, and decodes it to regenerate original information series.
The Viterbi decoding realizes the decoding with three processing, processing for obtaining difference (branch metric) between the received code sequence and the expected code sequence, processing for repeating ACS (Add Compare Select), and trace back processing for finally decoding data. Generally, a method for obtaining the branch metric with Hamming distance is referred to as a hard decision method, and a method for obtaining the branch metric with Euclid distance is referred to as a soft decision method. While it is an advantage that electric power consumption is small because amount of calculation of the hard decision method is less than that of the soft decision method, the capability of error correction is lower than that of the soft decision method. Thus, a receiver using the soft decision method whose capability of error correction is high is usually adopted in consideration of performance-improvement of the receiver.
In recent years, the high throughput and the high capability of error correction with the small electric power for transmission are required in UWB method (Ultra Wide Band) communication method adopting MB-OFDM (Multi Band-Orthogonal Frequency Division Multiplex) method which is estimated to become widely used as PAN (Personal Area Net work). And, this communication method is estimated to be implemented for a mobile terminal, so that high capability of error correction and also suppression of the electric power consumption are required.
By the way, as described above, because the amount of calculation of a receiver of the soft decision method is large, the electric power consumption is also consequently large. While the amount of calculation of a receiver of the hard decision method is relatively small, the capability of error correction is also relatively low.
Various approaches have been performed in order to resolve such dilemma.
Patent Document 1 (Japanese Patent Laid-Open No. 2003-249860) discloses the technology which suppresses the electric power consumption using the soft decision method. FIG. 17 illustrates a conceptual diagram of such technology. When the convolution code is decoded, the received signals such as three series of Output A, Output B, and Output C illustrated in FIG. 16 are converted to three series of data having the width in bit corresponding to the number of bits of the soft decision level as illustrated at the left side of FIG. 17. This data is digital multi-value data which expresses amplitude of the received signal. When the branch metric is obtained using this digital multi-value data, the technology of Patent Document 1 changes bits of the digital multi-value data according to the amplitude of the received signal, then obtains the branch metric. The change of bits is specifically performed by shifting downward each series of bit line forming the digital multi-value data by n bits corresponding to the amplitude, and fixing the upper n bits to “0” so that the effective width in bit after bit-shifting is smaller as the amplitude is larger. Thereby, because the digital multi-value data illustrated at the left side of FIG. 17 is converted to the data illustrated at the right side of FIG. 17, it is possible to reduce the amount of calculation for calculating the branch metric and suppress the electric power consumption when the amplitude is large.
Patent Document 2 (Japanese Patent Laid-Open No. 2006-086761) discloses the technology for selectively using the hard decision method and the soft decision method according to the quality of the received signal. In this technology, the hard decision method is used for the decoding when the quality of the received signal is good, and the soft decision method is used for the decoding when the quality of the received signal is not good. Thereby, when the quality of the received signal is good, it is possible to reduce the amount of calculation, and decrease the electric power consumption.
However, the Patent Document 2 does not specifically disclose the estimation method of the quality of the received signal.
And, in the technology of the Patent Document 1, the receiving condition is good when the amplitude of the received signal is large, and the receiving condition is bad when the amplitude of the received signal is small, so that the amplitude is used as an indicator of the receiving condition of the received signal. It may not be necessarily appropriate that the receiving condition is equally determined according to the size of the variable amplitude of the received signal regardless of the superiority or the inferiority of the transmission line quality or the channel quality.
And, because the technology of the Patent Document 1 narrows the width in bit to reduce the amount of calculation by shifting the bit line for each series of the digital multi-value data, it is necessary to add a circuit for shifting the bit line. Because the shifting circuit also consumes the electric power, the suppression effect of the electric power consumption which is obtained by reducing the amount of calculation is smaller.
Further, because the technology of the Patent Document 2 needs to provide two different methods of decoders, the implementation area is large. It is disadvantage particularly for a mobile terminal.